The window alarm is one well-known form of perimeter alarm and employs a thin metal foil strip cemented around the periphery of a window pane such that breakage of the pane causes breakage of the foil and interruption of electrical current which is caused to flow through the foil to thereby cause alarm actuation. Although this relatively simple configuration generally is quite effective, installation costs are quite high as are subsequent maintenance costs occasioned by damage to the foil such as by window cleaning or by inadvertent rubbing against the glass. Moreover, an alarm condition can be avoided by cutting through the protected glass without disturbance of the area containing the foil.
Another known form of window breakage detector employs a passive sensor affixed to a window surface and which senses vibration, shock or high frequency sound which can occur upon breakage of the protected window. In such passive systems, the nature and characteristics of a sensible signal produced upon window breakage cannot always be foreseen. As a result, such passive systems do not usually exhibit good alarm sensitivity and reliability for many commercially realistic installations.